Make sure your TV audio output is set to surround mode.
The rear speaker volume can be adjusted. Here is a link to the manual if you don't have one.
http
/www.vizio.com/sb3851c0.html
The rears are not supposed to be at the same volume of the fronts except when the program requires them to be. Most over the air programming will not have much rear channel information (a lot won't have any). If the program has no surround then turning the rear volume up won't increase what isn't there. If there is some increasing the rear level will make it more prominent.